Google Reader's Feed Unique ID

I'm building an app using Google Reader's API. The XML for a particularly feed (aka entry) returned by Google Reader looks as follow:

<entry gr:crawl-timestamp-msec="1294812847894">
  <id gr:original-id="">tag:google.com,2005:reader/item/fc8597a0dcb83203</id> 
  <category term="..." scheme="http://www.google.com/reader/" label="read" /> 
  <category term="..." scheme="http://www.google.com/reader/" label="reading-list" /> 
  ...
</entry>

Question: is the value of the id element (i.e. "tag:google.com,2005:reader/item/fc8597a0dcb83203") globally unique (i.e. across all subscriptions and all Google Reader users)? If not, what should constitute the globally unique id for a feed?

Solution

Yes, the ID is globally unique. This is why Reader generates its own IDs (and keeps the IDs from the original feed (if any) in the gr:original-id element), to ensure that uniqueness.